1. Field of the Invention
The present invention relates to an automatic transmission apparatus for motor vehicles which has a creep prevention mechanism, and more particularly to a creep prevention control device for the creep prevention mechanism.
2. Description of the Prior Art
Many motor vehicls or automobiles are equipped with automatic transmission apparatus in recent years. Although several types of automatic transmission apparatus are available for use on motor vehicles, most automatic transmission apparatus in use today comprise a torque converter and a gear mechanism having a plurality of selectively usable gear trains. The gear trains are automatically selected according to patterns assigned respectively to shift lever positions such as a drive position D, a manual low position L, a reverse position R, a neutral position N, and other positions. No gear train is selected in the neutral position N of the shift lever.
It is known that when the shift lever is in the drive position D while the engine is idling, some motor vehicles with such automatic transmission apparatus are slowly moved, a phenomenon known as so-called creep. The automatic transmission apparatus for such automobiles employ a clutch coupled to a first gear train for controlling power transmission through the first gear train. The clutch is constructed such that it can transmit engine power by being supplied with oil under pressure. The creep is caused since this clutch is not completely disconnected. The automobiles suffering the creep are disadvantageous in that the engine is subjected to a load when the motor vehicle is at rest.
To prevent the creep from being caused, there has been developed an automatic transmission apparatus having a creep prevention mechanism for releasing the pressure of the working oil from the first-gear-train clutch when the engine is idling and the motor vehicle is at rest. In such an automatic transmission apparatus, however, it is difficult to effect creep prevention at desired timing at all times. Another drawback with the developed automatic transmission apparatus is that there is a wide difference between an engine idling speed at the time the transmission apparatus is in a creep prevention mode and an engine idling speed at the time the transmission apparatus is not in the creep prevention mode.